The present invention relates to a method for monitoring degree of cure, and other properties of a coating or film cured by cationic polymerization using fluorescence methodologies.
Fluorescence spectroscopy has received considerable interest as an analytical tool due to its high sensitivity, selectivity and non-destructive characteristics. In addition, remote sensing is readily available using fluorescence methods through the use of fiber-optic cables to transmit optical signals to and from the analytical site in real time. Fluorescence spectroscopy has proven to be an extremely useful analytical tool in polymer chemistry. The technique has provided valuable information on the mechanisms and kinetics of polymerization, curing and crosslinking as well as oxidation, degradation and stabilization. The technique of fluorescence spectroscopy has been particularly useful in elucidating polymer properties such as their macro or supermolecular structure, molecular weight, viscosity and permeability.
Fluorescence probes as hereafter defined, have been used to study polymerization kinetics. For example, Paczkowski and Neckers, "Following Polymerization Kinetics of Multifunctional Acrylates in Real Time by Fluorescence Probe Methodology, Macromolecules, 25, 548-553 (1992) discusses the use of fluorescing probes such as dansylamide to follow the kinetics of polymerization and post-irradiation processes of multifunctional acrylates in real time. Other articles describe the use of fluorescence probes in polymerization studies of acrylates such as Paczkowski and Neckers, "Developing Fluorescence Probe Technology for Monitoring the Photochemical Polymerization of Polyolacrylates," Chemtracts-Molecular Chemistry, vol. 3, 75-94 (1992); Paczkowski and Neckers, "New Fluorescence Probes for Monitoring the Kinetics of Laser-Initiated Polymerization," JPS: Part A: Polymer Chemistry, vol. 31, 841-846 (1993); and Zhang, Kotchetov, Paczkowski and Neckers, "Real Time Monitoring of Polymerization Rates of Polyacrylates by Fluorescence Probes II. Effect of Depth of Polymerization for a Bleaching Photoinitiator System," The Society for Imaging Science and Technology, vol. 36, No. 4, 322-327, July/August 1992.
Various fluorescent probes have been developed recently for monitoring the progress of polymerization or final degree of cure of photocurable coating formulations cured by free radical polymerization. See, for example, U.S. Pat. No. 5,606,171. Typical fluorescent probes used for the free radically cured coating formulations contain basic amino functionality in their structure which is necessary to afford high fluorescence efficiency and large shift of the fluorescence spectrum with change in microviscosity and micropolarity of the probe environment. These probes are not useful for typical cationic cures because they interfere with the polymerization reaction. Thus, there is a need for a fluorescence probe which is useful in a process for determining degree of cure of a polymeric coating or film cured by cationic polymerization.